The present invention relates to an adhesive preform cover and, in particular, to an adhesive preform cover for an optical device and a method for making such cover.
Many approaches have been tried for packaging electronic devices for protection against external hazards, such as handling and other mechanical damage, environmental factors, chemical attacks, and other potentially adverse elements. Depending on both the functional and aesthetic requirements, these electronic devices are typically packaged in several levels of packaging. The outermost level is most likely a housing or enclosure for the equipment of which such devices are a part.
Generally, a useful electronic device, such as electronic circuit or integrated circuit, is packaged within a small package or module providing the first of at least several levels of protection. Electronic devices such as semiconductor devices are often protected by solid organic encapsulation. When several of these packaged electronic devices are put together as a functional unit, such as in an electronic circuit module or on a printed circuit board or other substrate, they are often protected with an exterior lid, cover or other enclosure to form a protective housing. These exterior lids or covers may be attached with adhesive, solder, or by mechanical fasteners, such as screws, bolts and clips.
In some applications, an electronic device at the semiconductor device level may not be able to reliably be encased in a solid encapsulant because of the adverse influence of stresses induced in the device owing to direct contact with the encapsulant. In other applications, the use of the encapsulation may be too costly. In still other applications, there may be a need for a lid or cover that is electrically conductive so as to provide shielding against electromagnetic interference (EMI) which may originate in the covered device or which may originate externally and to which the covered device may be susceptible. In this type of EMI-resistant application, the lid must be electrically conductive and must also be connected to the electrical ground of the electronic device. This requirement cannot be easily met with either an insulating organic encapsulant which does not provide shielding or with a conductive encapsulant which is likely to electrically short the electronic device or the conductors connecting thereto. Even the use of an electrically conductive lid that is soldered in place may be inconvenient or impractical because of the adverse effects on the devices that result from the high temperatures required for making soldering attachments. In addition, if one needs to rework the soldered module, the de-soldering operation may also cause overheating or other damage or the inadvertent de-soldering of other electronic elements inside of the package.
In fact, most of the electronic devices utilized in aerospace, military and other high reliability applications make use of a hermetically-sealed lid to prevent moisture and other adverse elements from affecting or damaging the electronic components employed therein. However, true hermetically-sealed packages are very expensive to fabricate, especially for optical devices requiring an optically transparent cover. Most high-reliability hermetically-sealed packages employ either metal soldering or brazing for lid attachment, especially for applications requiring an electrically conductive housing for EMI protection. In those applications where an insulating lid or cover must be employed, high temperature glass seals are often utilized. In order to prevent damage to the electronic devices from the high-temperature processing necessary to form the glass seals, the packages and lids must be heated up locally only along the rim of the package and lid. As a result, the processing time is long and the work of attaching the protective lids is delicate. In addition, the materials employed in both the glass seal and lid must have respective coefficients of thermal expansion (CTE) that are matched to that of the electronic substrate or package to which they attach. This additional requirement of matching the respective CTEs of the substrate, sealing material, and lid, all increase the difficulty of package design and the cost of the finished device. In general, the cost of both the materials and the processing of matched-CTE packages are prohibitive for commercial electronics products for general use, such as consumer electronic products.
Electronic package lids and covers are used, however, to a certain extent in commercial electronics products where required to achieve necessary performance parameters. For example, optical devices that must receive light in the visible, ultra-violet and/or infrared spectra cannot be simply encapsulated and so are protected by a lid that is transparent to such light. These lids are generally attached with adhesive.
Conventionally, adhesive in the form of dispensable paste or a die-cut preform is applied to the device or to the lid or cover immediately before or as part of the lid attachment bonding process. In certain cases, for, example, when the number of lid attachments is high, lids are pre-coated with adhesive or with die-cut adhesive preforms that will flow and cure when applied under heat and pressure conditions during the lid attachment process. However, the cost of adhesive pre-coating and die cut adhesive preform application to lids and covers is still quite high, in part due to the number of steps required and the handling of individual lids and even individual adhesive preforms. Adhesives in liquidous form are typically dispensed with a programmable automatic dispenser or are roller-coated onto the sealing areas of each lid, and are then subsequently dried or B-staged at a temperature and for a time substantially lower than the specified curing temperature and time for the particular adhesive. The liquidous adhesive is thus changed into a solid state either through solvent evaporation or chemical cross-linking of the adhesive during this drying or B-staging.
U.S. Pat. No. 5,056,296 issued to Ross et al and entitled xe2x80x9cIso-Thermal Seal Process for Electronic Devicesxe2x80x9d discloses an apparatus and process wherein the apparatus heats the lid, the package and the surrounding thermosetting adhesive so that they all attain an isothermal condition, i.e. a uniform temperature, before the lid is mated to the package in the bonding process. The Ross et al patent describes the pre-sealing isothermal condition as necessary to prevent differential air pressure between the inside and outside of the package that can cause xe2x80x9cblow-outxe2x80x9d-induced pinholes along the bond line provided by the sealing thermosetting adhesive if the parts are brought together and then are heated. Because of the time required to raise the temperature of the lid and the package, perhaps several minutes to achieve uniform temperature, the Ross et al process would appear able to achieve significant quantity production only when applied in a batch processing of lids, which often is impracticable. Moreover, because of the long heating time, the Ross et al process would seem to require a slower curing adhesive so as to avoid gelling or partial curing of the pre-heated adhesive before attachment of the lid to the package, thereby also extending the post-attachment curing time of the adhesive and further reducing the ability to achieve quantity production.
Thus, there is a need for an efficient method of pre-coating and pre-applying adhesive preforms onto lids and covers for optical devices to provide a cost-effective solution for protecting such devices. It is also desirable that such method lend itself to automated processing and that the adhesive of the preform be removable at a temperature and an applied force that will not damage either the optical/electronic components inside the package and/or the substrate to which they are attached.
There is also a need for such lids and covers that also provide shielding against EMI and that can be attached at a temperature substantially below the general soldering temperature of about 220xc2x0 C. It is also desirable that the adhesive employed therein is electrically conductive and bonds essentially instantly upon reaching the bonding temperature, and that the lids or covers so attached be removable at a temperature below the general soldering temperature so as to eliminate the possibility of thermally-induced damage to or misalignment of components inside the package.
According to the invention, a cover for an optical device comprises a planar sheet of an optically transmissive material having a central region shaped and sized to be at least as large as an optical device to be covered and having a peripheral region surrounding the central region. A closed bonding pattern of adhesive in the peripheral region of the planar sheet of optically transmissive material has at least one gap therein, the width of the gap being sufficiently small as to close when the adhesive is caused to melt flow.
In another aspect, the method of the present invention for making optical covers comprises:
providing an optical substrate;
depositing a pattern of adhesive including a plurality of cover bonding patterns on the optical substrate; and
singulating the optical substrate into a plurality of optical covers each of which has one of the cover bonding patterns of adhesive attached thereto.
According to a further aspect of the invention, a method for making an optical cover comprises:
providing sheet of an optical substrate material that is one of optically transmissive and optically transparent;
depositing a pattern of adhesive including a plurality of cover bonding patterns on the sheet of optical substrate material;
drying or B-staging the deposited pattern of adhesive; and
singulating from the sheet of optical substrate material at least one optical cover which has one of the cover bonding patterns of adhesive attached thereto.